Growth of practically all the organs and tissues of the body takes place through a process of interstitial multiplication and increase in size of the individual cells that go to make up their structure. In order that these changes may take place, it is necessary that there be a plastic medium and sufficient space for expansion. An organ like the liver or a tissue-like muscle possesses such prerequisites, and they increase in size by interstitial growth. Even in a tissue of such firm consistency as cartilage, it is found that its increase in size is due to the multiplication and growth of the individual cells. This form of interstitial growth also takes place in the cartilaginous forerunner of bone, but it is questionable whether it occurs after the laying down of the lime salts and a completion of the transition into bone. When the literature on the subject of interstitial